Photographic medium having a binder-free silver halide layer and methods of preparing same



United States Patent PHOTOGRAPHIC MEDIUM HAVING A BINDER- FREE SILVER HALlDE LAYER AND METHODS OF PREPARING SAME Melvin H. Saxe, Lexington, and Edward Hartonni, Westwood, Mass., assignors, by mesne assignments, to Technical Operations, Incorporated, a corporation of Delaware No Drawing. Filed Dec. 11, 1962, Ser. No. 243,746

34 Claims. (Cl. 96-67) This invention relates to photography and more particularly to novel sensitized photographic media and process for preparing same.

Photographic media formed of a substantially homogenous mass of contiguous microcrystals of silver halide in a binder-free layer on a suitable substrate, and a process for manufacturing such media by vacuum evaporation techniques are disclosed in French Patent No. 1,267,623, granted June 12, 1961, to Technical Operations, Incorporated. Media of this type can be distinguished from conventional emulsion type photographic film and other photographic media not only by their structure and method of manufacture, but also for some purposes by their high acutance, high resolution and quick developability. However, such media generally exhibit native sensitivities, in terms of ASA speed ratings, for example in about the range of 1X10 to l l0 Such media may be photographically sensitized by treatment of the open surface of the microcrystalline layer, i.e. the surface of the layer which is not contiguous with the substrate.

Where the sensitivity of such media is enhanced by such treatment of the open surface of the silver halide layer, certain problems arise. Particularly, when it is desired to treat such media with both chemical and optical sensitizers, for example to render the media panchromatic as well as to increase either speed or density, the optical sensitizer and chemical sensitizer are generally antagonistic. Consequently, either sensitivity to white light is reduced, or optical sensitization is poorly, if at all, achieved. Additionally, some chemical sensitization procedures are not compatible with other treatments. For instance, surface sensitization can be achieved by depositing a sensitizer material, such as an elemental metal, upon the open surface of the microcrystalline silver halide layer. Subsequent treatment with an oxidative etch, such as potassium dichromate, normally quite effective in increasing the density scale of the silver halide, will destroy the sensitization achieved by the metallic deposit. Similarly, if the silver halide mircrocystalline layer is first optically sensitized by application thereto of a dyestufi', the use of oxidative etch destroys the optical sensitization.

The present invention is directed toward the sensitization of mircocrysalline, binder-free, photographic media and has for its principal object the provision of a novel method for sensitizing media of this type and the products of such method.

More specifically, other objects of the present invention are:

(1) To provide a sensitized photographic medium comprising a microcrystalline, binder-free layer of silver halide deposited on a surface of a support element and having a sensitivity stratum disposed at the interface of said layer and said surface;

(2) To provide a method of photographically sensitizing such a photographic medium by formation of an image-forming sensitivity stratum on the surface of the microcrystalline binder-free silver halide layer adjacent to the substrate, thereby leaving the opposite surface of the layer available for sensitization by a similar or other method;

3,219,449 Patented Nov. 23, 1965 (3) To provide a novel sensitized photographic medium;

(4) To provide a novel method of sensitizing such photographic media whereby sensitization can be achieved by treatment with two or more sensitizers or sensitizing methods, at least one of which is antagonistic to the other or others when applied to the same surface of such a medium;

(5) To provide such a medium which is sensitized at both surfaces of the silver halide layer so that sensitizations are achieved, although the sensitizing of each surface may be by different procedures with different materials.

Other objects of the invention will in part be obvious and will in part appear hereinafter. The invention ac cordingly comprises the processes involving the several steps and the relation and order of one or more of such steps with respect to each of the others, and the products and compositions possessing the features, properties and relation of elements which are exemplified in the following detailed disclosure and the scope of the application all of which will be indicated in the claims.

The present invention is intended to provide a substrate mounted microcrystalline, binder-free, silver halide photographic layer which is sensitized at the layer-substrate interface. Additionally such a medium is therefore sensitizable at the open surface of the layer by yet one or more other sensitizing procedures and materials without antagonistic effect between the sensitizers. Generally, the formation of such a medium is achieved by providing the usual photographically inert substrate, or support sheet, and distributing upon one surface thereof a material which will sensitize, either optically or chemically, a microcrystalline binder-free silver halide layer. The latter is then formed, as by vacuum evaporative techniques, by deposition of the microcrystals in a substantially homogenous binder-free stratum on top of the substrate surface hearing the sensitizer. This creates, at least adjacent the interface of the substrate surface and the binder-free stratum, an image-forming sensitivity stratum, ie a stratum which enhances the image-forming characteristics of the silver halide.

The foregoing process provides a novel and basic photographic medium which possesses several advantages. For instance, Where sensitizing materials are deposited, for example as a stratum, upon the outer or open surface of a binder-free microcrystalline silver halide layer, the thickness of such a sensitizer stratum renders it susceptible to mechanical damages. Further, when such a stratum is exposed to the ambient atmosphere, it is equally susceptible to contamination by or chemical reaction with airborne substances. In either instance, the sensitization achieved may then be impaired. The structure of the basic medium formed by the process of the present invention provides greater stability toward mechanical and chemical disturbance inasmuch as the sensitivity stratum lies in a protected position between the substrate and the silver halide layer.

The basic photographic medium of the present invention may also be readily modified to provide some unique effects. Because of the positioning of the sensitivity stratum, provided by a first sensitizing material, between the substrate and the silver halide layer, the outer or open surface of the latter can additionally be sensitized Without materially affecting the sensitization previously achieved. Thus, a large variety of combination sensitizations can be produced. In addition, depending upon the purpose for which the medium is to be employed, control of the thickness of the silver halide layer also yields some significant modifications. It is desired to keep the silver halide within a thickness range of about 0.1 to 0.5 micron inasmuch as the optimum photographic parameters such as gamma, density and sensitivity, are found within this range. Thin layers such as 0.1 micron to 0.2 micron are preferred in some modifications of the invention and thicker layers, e.g. 0.2 to 0.5 micron are desirable in other modifications for reasons pointed out hereinafter.

For instance, the sensitization achieved by depositing a first sensitizer onto the substrate and subsequently depositing a contiguous microcrystalline layer of silver halide over the sensitizer, is believed to be a surface effect. In other words, the sensitivity stratum is believed to be confined to the interface between the substrate and the silver halide and probably does not extend more than a fraction of a micron (less than 0.1 micron) into the silver halide layer depending upon the smoothness of the substrate surface. Where the silver halide layer is relatively thick, e.g. about 0.5 micron, it is then apparent that the surface sensitization of the silver halide substrate interface will not increase the sensitivity of the medium with respect to the outer surface of the silver halide layer. Thus, the present invention contemplates a modification of the basic photographic medium in which diverse sensitizers are respectively applied to opposite surfaces of the microcrystalline silver halide layer.

Binder-free microcrystalline silver halide layers can be chemically sensitized to provide two different types of media, one of which is normally developable to provide a positive image, and the other of which is similarly developable but provides a negative image.

By providing a sensitizing stratum between a silver halide layer of predetermined thickness and the substrate, and then treating the open surface of the layer with the same or another sensitizer, one obtains a photographic medium which has quite unusual properties. Such a medium, if exposed to form a latent image and then developed at the open surface with a developing agent that does not materially dissolve the silver halide, will yield a developed surface image of one type. Such surface silver images are amenable to transfer from the silver halide layer to a receiving sheet by adhesion. This transfer may be readily effected because the adhesion between the developed silver and the underlying undissolved silver halide is quite poor. Thus, the surface silver image at the second sensitized surface may be removed by transfer, may be destroyed by mechanical abrasion for example, or may be removed by the use of an oxidative etch such as a solution of potassium dichromate. However, if the medium is sufliciently thick e.g. 0.2 micron to 0.5 micron, so that the first development has not materially affected the sensitization provided by the sensitizing stratum at the interface of the silver halide layer and substrate, the medium may now be further developed in a developing reagent, such as an internal developer which contains for example a quantity of silver halide solvent, to remove silver halide overlying the sensitivity stratum so that the latent image at or adjacent the latter can be developedinto a silver image of, if desired, the other type. It will be apparent tht the dual image thus obtained by a single exposure and subsequent developments, can be a combination of a positive and a negative, a pair of positive images, or a pair of negative images. In one typical form, by using a transparent substrate, a positive image may be obtained at the outer sensitized surface which image is then transferrable to an opaque reflective transfer sheet to provide a positive reflection print. The image formed at the silver halide-substrate interface can be also a positive. Thus, both a positive transparency and a positive reflection print can be formed from the same medium.

In another modification of the present invention, the basic medium heretofore disclosed is again sensitized on its outer surface to provide a dual sensitization. However, in this form the two sensitizers differ in that one is a chemical sensitizer which increases the sensitivity of the medium to white light, while the other is an optical sensitizer which extends the useful range of the medium in terms of frequency response. In such instances, it is preferred that the silver halide layer be sufficiently thin (e.g. about 0.1 to 0.2 micron) so that while the two sensitizations do not chemically interact with one another, the medium can be developed as by an internal developer to provide a single image adhering to the substrate.

In the preferred form of the invention, while the sensitivity stratum can be formed by depositing the sensitizing material upon the substrate as by precipitation from solution, it is preferred that the sensitivity stratum be formed by deposit of sensitizer from its vapor state upon the substrate as by vacuum evaporative techniques and by the deposit of the silver halide layer over the sensitizer by the same technique. The sensitization of the outer surface of the silver halide layer can be achieved in a variety of ways, for instance by a third evaporation deposition, by deposit from solution, by treatment with sensitizing solutions, and other like techniques.

Typical photographic media according to the present invention may be prepared by vacuum evaporation techniques similar to those described in the aforementioned French Patent No. 1,267,623. A basic machine of the kind useful in applying such techniques to form media of the present invention is illustrated and described in the book Vacuum Deposition of Thin Films by L. Holland, published by John Wiley & Sons, New York City, 1948, pp. 7-8.

Vacuum coating apparatus of this type is well known and usually comprises an evacuatable container, such as a bell jar, and known means for evacuating the latter to a predetermined and controlled ambient. pressure which is preferably less than 1 l0- mm. of Hg. The latter appears to be the maximum pressure at which reasonably desirable film formation from silver halides can be achieved. Included within the bell jar is a crucible or boat which is intended to carry the material which is to be evaporated. Means for heating the boat are usually included, and in one form of the apparatus the boat is made of, for instance, tungsten provided with leads to a source of electrical power so that the crucible can be heated electrically. By this method the temperature of the evaporating material can be readily controlled. In other known forms of vacuum evaporation apapratus, the starting material may be heated by electrical induction, for example. The apparatus also includes means for holding substrate material, such as a support sheet, at a predetermined location spaced from the boat within the bell jar wherein preferably one surface of the substrate faces the boat, thus insuring that the stream of vapor from the evaporating material in the boat will strike the substrate surface and condense thereon to form a film or layer according to the well-known principles of evaporation techniques.

In one method of using this apparatus a quantity of evaporable sensitizer is placed within the container, for example, at a first location and a suitable substrate sheet is placed Within the container at a second location displaced a predetermined distance from the first location. The container is pumped down to achieve an operating pressure, and the sensitizing material brought to a temperature at which evaporation thereof will occur.

The resulting vapor, as well known in the art, will transfer to and be deposited upon the substrate surface and can readily be controlled to provide a layer of substantially uniform thickness. The concentration of material so deposited is determined according to such factors as the heated sensitizers vapor pressure which is temperature dependent, and the time during which evaporation occurs. Of course, the substrate may be coated in either batch or continuous manner by this technique or by other techniques such as precipitation from solution.

After the sensitizing material has been deposited in a substantially uniform manner to the desired concentration upon the substrate, a microcrystalline binder-free silver halide layer of predetermined thickness is then deposited thereon. This may be accomplished in the same apparatus, for instance, by leaving the sensitizer-coated substrate in its position, replacing the container of sensitizer with a container of silver halide, again pumping down the operating pressure, and raising the silver halide to an evaporation temperature. Alternatively, in a continuous process, the sensitized coated substrate in strip form can be continuously transported out of the evaporative stream of sensitizer and through another position within the same ambient reduced pressure. Opposite this latter position there is located a second crucible in which silver halide has been raised to a temperature of evaporation and the vapor stream therefrom is then directed to- Ward and condenses upon the moving sensitizer coated substrate. In either process, the thickness of the silver halide layer is readily controlled as by adjustment of the evaporation temperature, the evaporation time (in batch processing) or speed of movement of the substrate (in continuous processing), and the distance between the silver halide source and substrate, or any of them. The layer thus provided exhibits a density which appears quite close, i.e. within about to that of a solid silver halide crystal. As taught by the aforesaid French patent, the density ratio of polymicrocrystalline layers of this type to solid crystals is approximately 95%.

Among the silver halides which are useful in evaporative processes to form the media of the present invention are silver bromide, silver chloride and silver iodobromide. Exemplary support or substrate materials upon which the doped layer is deposited include many diverse materials, either moisture impervious and pervious, e.g. vitreous substances such as glass; fibrous materials such as paper and composition board; natural polymers such as gelatin; synthetic resins including particularly polyethylene terephthalate, polyvinyl chloride, copolymers of vinyl acetate-vinyl chlorides, polystyrene and many other polyesters, polyamides and the like; and cellulose esters such as cellulose acetate, cellulose propionate and the like.

As materials which can be evaporated onto a surface to provide a sensitization of the microcrystalline binderfree silver halide are the normally (i.e. under standard conditions of temperature and pressure) solid elements other than the halogens, selected from the group of elements of the periodic table classified in Groups I through VI and VIII. Examples of chemical elements which can be vapor deposited on the outer surface of a micro- 5 crystalline, binder-free silver halide photographic medium to increase the photographic sensitivity thereof, and with which, in each case, there can be obtained upon standard exposure and development a normal (i.e. a negative) image, a solarized (i.e. direct positive) image or both, are set forth in tabular form in Table I following. In this table, the type of image ultimately achieved as a function of coating level or concentration is set forth in two columns entitled respectively Normal" and Solarized and a column also has been included entitled Pegged. Where all three, or two of three, of the above can be achieved, fogging is obtained with the thickest (or thicker of the two) coatings of the element involved. Inasmuch as fogging is undesirable, detracting from the photographic qualities of images, conditions wherein fog occurs should be avoided generally. Where normal and solarized images are obtained with a given element, the thicker layer, i.e. the heaviest concentration, yields the solarized image. In general, Where a normal image is obtained the layer of sensitizer material is not substantially greater than 10 atoms per square centimeter of adjacent surface of the photographic silver halide. Solarized images are generally achieved Where the concentration of sensitizer is between approximately 10 to 10 atoms per square centimeter of adjacent silver halide surface. Layers in yet heavier concentration result in fogging.

The last three columns set forth the approximate pressure in millimeters of mercury, time used for achieving appropriate deposition of the sensitizer vapor, and either the voltage or current through a crucible, at which volt age or current, satisfactory vapor deposition of the element is obtained. The filament used in most cases in Table I is tungsten and has approximate dimensions of 0.75 x 2.25 X 0.002 inch. In some cases, a tantalum filament or other filament material is preferably employed to avoid having a filament with which the starting material reacts chemically. For instance, for vapor de- TABLE 1 Element Results obtained as function Evaporation Conditions for Normal of coating level Image Normal solarized Pressure Time Volts or Image Image Fogged (mm. of Hg (see) Amps 3 amps.

amps. 90 amps. 2 amps.

25 amps. 43 amps. volts. 55 volts. 70 amps. volts.

.75 volts. .75 18 amps.

I 45) volts.-

75 amps.

amps. 76 amps. 45 volts. 0.73 volt. 50 amps. 44 amps. 45 volts. 28 amps. 0.4 volt. 47 volts. 40 volts. 73 amps.

positing antimony and bismuth a tantalum filament is preferred. The teachings of Holland at pages 110-114 in his aforesaid book can be followed in many cases, particularly in the use of a boat form of filament.

To obtain the results set forth in Table I starting quantities of milligrams or less of each element are employed.

The media sensitized to .provide normal images, upon exposures of relatively short duration, for example second, and subsequent development will of course yield negative images. However, some of these media, if exposed instead for relatively long periods, for example 30 seconds, will exhibit a tendency to solarize. This solarization tendency can be inhibited by treatment, as by immersion, in a solution of 10 milligrams of ammonium chloroiridite per liter of water and subsequent drying before exposure.

Where the elemental material is deposited on the substrate for sensitizing a silver halide layer subsequently deposited thereon, the elements of the above table are also effective as sensitizers although the use of the ammonium chloroiridite solution is often precluded. In such instances, because certain of the elements, such as chromium, selenium and sulfur, display a strong tendency to yield a solarized image even when treated with the solution. Their usage to provide negative-image forming sensitization by deposit on the substrate is not recommended unless the substrate is pervious to the solution and the silver halide substrate outerface can thus be treated.

Deposition of elemental materials can also be achieved from solution. This is of particular value in such cases as the use of sulphur and selenium because these latter are difficult to control in a vapor deposition process owing to their relatively high vapor pressures at relatively low temperatures. A layer of sulphur, for instance, can be deposited from solution using a colloidal suspension of sulphur in dioxane, a quick evaporation vehicle identified in the Merck Index, 7th edition, page 387, published 1960 by Merck & Co., Inc. of New Jersey. Selenium and sulphur can be deposited, each from a solution in dimethyl sulfoxide (Merck Index, ibid., page 373). In each latter case a saturated solution is employed at room temperature, and the precipitant is washed with water with which dimethyl sulfoxide is miscible and in which the precipitant is relatively insoluble.

sary to after-treat the dye layer with water, apparently to provide proper aggregation of adsorbed dye molecules. This presents no difiiculty when the dye is applied as the second sensitizer to the outer surface of the silver halide layer. However, when applied to the substrate to form a sensitizing stratum disposed between the latter and the silver halide layer, it is necessary to provide a structure whereby water can be applied to the dye after the silver halide has been deposited thereon. This can be accomplished by employing a water-pervious substrate such as filter paper, or by providing a subbing surface on the substrate containing a substance which will liberate water as by the application of heat thereto, for instance a diatomaceous earth, a hydrated salt or the like.

Where the sensitizier is to be applied to either the substrate or silver halide layer open surface by other than an evaporative process, a wide variety of sensitizers is available for use, for instance, a host of dyes in aqueous or aqueous-alcohol solutions for optical sensitization, and many diverse chemically sensitizing materials. Included in the latter are organic compounds such as condensation products of alkylene oxides, various polyamines, polyvinyl resins, sulfoxides, complexed borane hydrides and many others. Among the useful chemical sensitizers are inorganic compounds such as copper halides, ammonium halides and acetate, salts providing hydroxyl ions in solution and many others.

In the examples set forth hereinafter wherein development processes are used, the surface developer employed is as follows:

Solution A Metol 0.50 Sodium sulfite (anhydrous) 19.50 Hydroquinone 1.87 Water to make 250 cc.

Solution B G. Sodium carbonate (anhydrous) 78.00 Potassium bromide 2.0

Water to make 1000 cc.

Solution C Gelatin, 1.25 g. Water to make 250 cc.

These are mixed to provide a final developing solution by combining equal quantities of Solution A and B and then adding a like equal quantity of Solution C to the mixture of Solutions A and B. The internal developer employed is formed from 60 ml. of the foregoing surface developer to which 3 ml. of 1% sodium thiosulfate solution is added. Some examples setting forth the process and products of the present invention, to be construed as exemplary and not as limiting, are as follows:

EXAMPLE 1 A sheet of subbed polyethylene terephthalate available commercially under the trademark Cronar from E. I. du Pont de Nemours & Co., Delaware, and in which the substratum is believed to be a water permeable colloid layer (described in US. patents, No. 2,627,088 or No. 2,698,242) is placed in a vacuum deposition apparatus of the type described at a distance of approximately 4 inches from a tungsten container having dimensions of approximately 0.75 x 2.25" x 0.002". A sample of less than 10 milligrams of gold is placed in the container. The apparatus is pumped down to achieve an operating pressure of approximately 2X10 mm. Hg and the tungsten container is heated by application therethrough of 2 amperes at 110 volts. After the gold is melted, the substrate is exposed to the vapor stream therefrom for a period of 5 seconds. This forms a layer of about 10 or slightly less (i.e. within one order of magnitude) atoms per square centimeter of the surface of the sheet. Im-

. mediately thereafter a new container of similar size is substituted for the first container and a sample of silver bromide of high purity, i.e. less than ppm. impuri ties, placed in the new container. The apparatus is again pumped down to achieve an operating pressure of approximately 1X10 mm. of mercury and the container raised to a temperature (about 625 C.) at which the silver halide is molten. Under these conditions, the process is carried on for about one minute to evaporate the silver halide and condense the vapors thereof as a substantially homogeneous layer of contiguous microcrystals of binder-free silver bromide until the layer is of substantially uniform thickness of about 0.3 micron.

A photographic medium formed under similar conditions as hereinbefore set forth in this example, but with no deposit of sensitizer, exhibits an inherent sensitivity of between 1X10" and 1x10"? The gold-sensitized medium when under similar conditions of exposure and development exhibits an increase in sensitivity over the unsensitized medium by a factor of about 2 to 5.

EXAMPLE 2 A gold sensitized basic photographic medium is prepared according to Example 1. However, the silver halide outer surface thereof is sensitized by application in the evaporator apparatus of another layer of gold deposited in an apparatus of another layer of gold deposited in an increased time (about 5.3 sec.) to achieve a concentration of between and 10 atoms per square centimeter of the silver halide surface on which it is deposited. The medium thus formed is exposed and then developed with a surface developer for seconds. This forms a surface silver image on the surface of the silver halide layer sensitized with the second deposit of gold. The surface developed medium is then washed with water and dried. Upon applying a tacky transfer medium such as a pressure-sensitive adhesive coated polymeric sheet (available for example as Scotch brand tape manufactured by Minnesota Mining and Manufacturing Co.) to the silver image and stripping the tape from the medium the surface silver image is substantially completely removed leaving the medium substantially silver image free. Upon further development of the remaining medium in an internal developer, 21 second silver image is formed at approximately the location of the first sensitized surface of the silver halide layer. Because of the disparity in the concentrations at the two silver halide surfaces the first silver image removed with the transfer sheet is a positive image, while the second formed silver image is a negative image which adheres well to the support sheet.

EXAMPLE 3 A gold sensitized basic photographic medium is prepared according to Example 1. However, the evaporation time with respect to the silver bromide is reduced to achieve a microcrystalline binder-free silver halide layer of approximately 0.2 microns. A sample of about 5 mg. of l,1-diethyl 2,2'-carbocyanine bromide (pinacyanol) is sublimated at a 20 pressure at a temperature of approximately 200 C. onto the open silver bromide surface of the basic medium. The evaporation is continued until there is a substantially uniform distribution of a visible layer of dye on the silver bromide surface in a concentration of about 1 microgram per 70 sq. mm.

A medium prepared accordingly in this manner, but without a first sensitizing layer of gold, when exposed to light corrected to 5500K and developed, gives no indication of increased sensitivity. However, upon wetting the dye surface of such a medium with pure water, upon the same exposure and development the film shows a red sensitivity of the order of 4 l0 and a white light sensitivity of about 4X 10*.

The basic medium, when sensitized by application of dye to its surface, upon similar exposure and development shows a minimum sensitivity to the white light of more than 4X 10 with no impairment of the previously mentioned red sensitivity.

While in the foregoing examples the sensitivity stratum is provided by deposit of gold in the appropriate concentration onto the substrate prior to deposition thereover of the microcrystalline, binder-free silver halide layer, it will be clearly apparent that other material can be used to achieve similar results, and particularly that other elements listed in Table I are appropriate for such use.

Since certain changes may be made in the above process and products without departing from the scope of the invention herein involved it is intended that all matter contained in the above description or shown in the accompanying drawing shall be interpreted in an illustrative and not in a limiting sense.

What is claimed is:

1. An image recording medium comprising a substrate element having a surface providing a recording area, a photographic layer of vapor deposited photosensitive silver halide microcrystals in substantially continuous phase supported upon said element and substantially covering the extent of said area, said layer being adhered directly to said substrate and said microcrystals being cohered directly to each other, said layer having a density less than that of said halide in solid crystalline form, said layer being a fraction of a micron in thickness and the surface portion of said layer over said area immediately adjacent the substrate element having been treated with a sensitizing material to provide an increased photographic sensitivity for said surface portion.

2. An image recording medium as set forth in claim 1, wherein said layer has a thickness of from about 0.1 to about 0.5 micron.

3. An image recording medium as set forth in claim 1, wherein said silver halide is silver bromide.

4. An image recording medium as set forth in claim 1, wherein said silver halide comprises a plurality of halides.

5. An image recording medium as set forth in claim 1, wherein said density is about of that of said halide in solid crystalline form.

6. An image recording medium as set forth in claim 1, wherein said thickness is about 0.3 micron.

7. A silver halide photographic element comprising a substrate sheet having a surface providing a recording area, and a photoresponsive layer having a stratum of substantially binder-free, vapor deposited silver halide microcrystals supported on said surface of said sheet and substantially covering said area of said sheet, said stratum having a thickness of a fraction of 21 micron and a density of less than that of said halide in solid crystalline form, said layer further including a photosensitizer for said halide applied and distributed substantially uniformly over the surface of said stratum over said area immediately adjacent the substrate sheet.

8. An element as set forth in claim 7, wherein said thickness is from about 0.1 to about 0.5 mciron.

9. A method of forming a silver halide photographic element, comprising evaporating silver halide under a high vacuum with said silver halide at a temperature in excess of its melting point, condensing the silver halide vapors over a surface area of a base member to form a layer of light responsive silver halide upon said base as a support therefor, said evaporation being conducted from a pool of molten silver halide, said evaporation and condensation being effected under substantially stable conditions of pressure and temperature to afford substantially uniform characteristics to the silver halide deposit, and wherein said silver halide is deposited to a thickness sufficient to cover substantially said entire area of said base member but no greater than a fraction of a micron, and said method further including the application of a silver halide photosensitizer to the surface of said silver halide layer immediately adjacent the base member.

10. A method as set forth in claim 9, wherein said thickness is between about 0.1 and about 0.5 micron.

11. A method as set forth in claim 9, wherein said photosensitizer is applied by vapor deposition thereof onto said surface area of said base member before evaporation of said silver halide.

12. A method as set forth in claim 11, said vapor deposition is effected under high vacuum.

13. A method as set forth in claim 9, wherein said photosensitizer is applied to said surface of said base member from a liquid medium containing said photosensitizer before evaporation of said silver halide.

14. A method of forming a photographic medium comprising the steps of distributing a first photosensitizing material for silver halide onto a substrate surface and condensing a substantially homogeneous mass of contiguous unichrocrystals of silver halide from the vapor state thereof in a substantially uniform binder-free layer overlying said material, and treating the surface of said layer remote from said substrate with a silver halide photosensitizing material, (a) the first mentioned photosensitizing material and its concentration in terms of quantity per unit area of silver halide surface, and (b) the second mentioned photosensitizing material and the treatment therewith, each of (a) and (b) improving the image producing qualities of said silver halide over its unsensitized state relative to speed, or spectral sensitivity, or ability to produce a solarized image.

15. A method of forming a photographic medium comprising the steps of condensing a substantially uniform stratum of a normally solid, photosensitizing material for silver halide from the vapor state thereof onto a substrate, and condensing a substantially homogeneous mass of contiguous microcrystals of silver halide from the vapor state thereof in a substantially uniform binder-free layer overlying said material, and treating the surface of said layer remote from said substrate with a silver halide photosensitizing material, (a) the first mentioned photosensitizing material and its concentration in terms of quantity per unit area of silver halide surface, and (b) the second mentioned photosensitizing material and the treatment therewith, each of (a) and (b) improving the image producing qualities of said silver halide over its unsensitized state relative to speed, or spectral sensitivity, or ability to produce a solarized image.

16. A method of forming a photographic medium comprising the steps of condensing a substantially uniformly distributed quantity of a normally solid first photosensitizing material for silver halide from the vapor state thereof onto a substrate, condensing a substantially homogeneous mass of contiguous microcrystals of silver halide from the vapor state thereof in a substantially uniform binder-free layer overlying said material, and treating the surface of said layer remote from said substrate with a silver halide photosensitizing material, (a) the first mentioned photosensitizing material and its concentration in terms of quantity per unit area of silver halide surface, and (b) the second mentioned photosensitizing material and the treatment therewith, each of (a) and (b) improving the image producing qualities of said silver halide over its unsensitized state relative to speed, or spectral sensitivity, or ability to produce a solarized image.

17. Method as defined in claim 16 wherein one of said first and second sensitizing materials is an optical sensitizer, and the other of said first and second sensitizing materials is a chemical sensitizer.

18. Method as defined in claim 16 wherein one of said first and second sensitizing materials is a material for so sensitizing the adjacent surface of said layer as to provide said surface with the capability of having a latent image thereat normally developable into a direct positive image, and the other of said materials being a material for so sensitizing the surface of said layer adjacent thereto as to provide the latter surface with the capability of having a latent image thereat normally developable into a negative image.

19. Method as defined in claim 18 wherein said one of said materials is condensed in a concentration of approximately to 10 atoms per square centimeter of the surface of said layer adjacent thereto, the other of said materials being distributed on the surface of said layer adjacent thereto in a concentration of less than approximately 10 atoms per square centimeter of the latter sursurface.

20. Method as defined in claim 16 wherein all of said materials and said layer are deposited by vacuum evaporation.

21. Method as defined in claim 16 wherein said second sensitizing material is applied from solution to the open surface of said layer.

22. A photographic medium comprising a substrate, first photosensitizing material for silver halide distributed on a surface of said substrate, a homogeneous mass of vapor deposited contiguous microcrystals of silver halide in a binder-free layer in contact with and overlying said first sensitizing material, and second photosensitizing material for silver halide 0n the open surface of said layer, the concentrations of both sensitizing materials in terms of quantity per unit area of silver halide surface being selected to improve the image producing qualities of said silver halide over its unsensitized state relative to speed, or spectral sensitivity, or ability to produce a solarized image.

23. A photographic medium as defined in claim 22 wherein said layer has a substantially uniform thickness within the range of about 0.1 to 0.5 micron.

24. A photographic medium as defined in claim 23 wherein said layer has a density of approximately within 10% of the density of a solid crystal of said silver halide.

25. A photographic medium as defined in claim 24, wherein said silver halide is silver bromide.

26. A photographic medium as defined in claim 24 wherein said silver halide is selected from the group consisting of silver bromide, silver iodide, and silver iodobromide.

27. A photographic medium as defined in claim 22 wherein one of said sensitizing material is an optical sensitizer, and the other of said sensitizing materials is a chemical sensitizer.

28. A photographic medium as defined in claim 27 wherein said layer has a substantially uniform thickness in a range between about 0.1 to 0.2 micron.

29. A photographic medium as defined in claim 28 wherein said layer has a density of approximately within 10% of the density of a solid crystal of said silver halide.

30. A photographic medium as defined in claim 28 wherein said silver halide is silver bromide.

31. A photographic medium as defined in claim 28 wherein said silver halide is selected from the group consisting of silver bromide, silver iodide, and silver iodobromide.

32. A photographic medium as defined in claim 21 wherein one of said sensitizing materials effects a sensitization of the adjacent surface of said layer as to provide said surface with the capability of having a latent image thereat normally developable into a direct positive image, and the other of said materials effects a sensitization of said layer adjacent thereto as to provide the latter surface with the capability of having a latent image thereat normally developable into a negative image.

33. A photographic method of forming multiple images comprising the steps of forming a photographic medium by distributing a substantially uniform stratum of first chemical sensitizing material onto a substrate, condensing a substantially homogeneous mass of contiguous microcrystals of silver halide from the vapor state thereof in a substantially uniform binder-free layer overlying said stratum, treating the open surface of said layer with a second chemical sensitizing material, exposing said medium to form a developable latent image therein, developing with a surface developer the sensitized open surface of said layer to form a surface silver image, transferring substantially all of said surface image by adhesion to a transfer sheet to leave said medium substantially free of silver image, and developing said medium with a developer containing a silver-halide solvent to form a second silver image at the surface of said layer previously sensitized with said first material.

34. A photographic method as defined in claim 33 in- 3,219,449 13 14 cluding the step of controlling the condensation of said OTHER REFERENCES silver halide so that the uniform thickness of said layer Nelson: Contact Potential Diflemnc Between Sensi is in a range between about to micron tizing Dye and Substrate, J. Opt. Soc. of America, vol. 46, No. 12, December 1956, pages 1016-1019. (Copy in References Cited by the Examiner 5 Scientific Library UNITE STATES PATENTS Goldberg et al.: ASD Tech. Report 61-332, August 2,945,771 7/1960 M f ld 117 1()6 1961 (AD. 264,061) (copy in Scientific Library) pages 3 FOREIGN PATENTS 26 48 and 49 547,06 9/1942 Great Britain. 10 NORMAN G. TORCHIN, Primary Examiner. 

1. AN IMAGE RECORDING MEDIUM COMPRISING A SUBSTRATE ELEMENT HAVING A SURFACE PROVIDING A RECORDING AREA, A PHOTOGRAPHIC LAYER OF VAPOR DEPOSITED PHOTOSENSITIVE SILVER HALIDE MICROCRYSTALS IN SUBSTANTIALLY CONTINUOUS PHASE SUPPORTED UPON SAID ELEMENT AND SUBSTANTIALLY COVERING THE EXTENT OF SAID AREA, SAID LAYER BEING ADHERED DIRECTLY TO SAID SUBSTRATE AND SAID MICROCRYSTALS BEING COHERED DIRECTLY TO EACH OTHER, SAID LAYER HAVING A DENSITY LESS THAN THAT OF SAID HALIDE IN SOLID CRYSTALLINE FORM, SAID LAYER BEING A FRACTION OF A MICRON IN THICKNESS AND THE SURFACE PORTION OF SAID LYAER OVER SAID AREA IMMEDIATELY ADJACENT THE SUBSTRATE ELEMENT HAVING BEEN TREATED WITH A SENSITIZING MATERIAL TO PROVIDE AN INCREASED PHOTOGRAPHIC SENSITIVITY FOR SAID SURFACE PORTION.
 33. A PHOTOGRAPHIC METHOD OF FORMING MULTIPLE IMAGES COMPRISING THE STEPS OF FORMING A PHTOTGRAPHIC MEDIUM BY DISTRIBUTING A SUBSTANTIALLY UNIFORM STRATUM OF FIRST CHEMICAL SENSITIZING MATERIAL ONTO A SUBSTRATE, CONDENSING A SUBSTANTIALLY HOMOGENEOUS MASS OD CONTIGUOUS MICROCRYSTALS OF SILVER HALIDE FROM THE VAPOR STATE THEREOF IN A SUBSTANTIALLY UNIFORM BINDER-FREE LAYER OVERLYING SAID STRATUM, TREATING THE OPEN SURFACE OF SAID LAYER WITH A SECOND CHEMICAL SENSITIZING MATERIAL, EXPOSING SAID MEDIUM TO FORM A DEVELOPABLE LATENT IMAGE THEREIN, DEVELOPING WITH A SURFACE DEVELOPER THE SENSITIZED OPEN SURFACE OF SAID LAYER TO FORM A SURFACE SILVER IMAGE, TRANSFERRING SUBSTANTIALLY ALL OF SAID SURFACE IMAGE BY ADHESION TO A TRANSFER SHEET TO LEAVE SAID MEDIUM SUBSTANTIALLY FREE OF SILVER IMAGE, AND DEVELOPING SAID MEDIUM WITH A DEVELOPER CONTAINING A SILVER-HALIDE SOLVENT TO FORM A SECOND SILVER IMAGE AT THE SURFACE OF SAID LAYER PREVIOUSLY SENSITIZED WITH SAID FIRST MATERIAL. 